Chemical milling process



Unitd States isnt O CHEIVHCAL MILLING PROCESS Lowell W. Bahe, Milwaukee, Wis., assignor to Allis- Chalmers Manufacturing Company, Milwaukee, Wis.

No Drawing. Filed Apr. 16, 1958, Ser. No. 728,782

8 Claims. (Cl. 41-42) This invention relates generally to the controlled erosion of metals and more particularly to a process of chemical milling whereby a finish and depth of cut are readily controlled.

In the arts of fabrication, mechanical milling operations have become very complex when the piece to be milled has an unusual shape, when it is thin and therefore structurally weak, when the area to be milled is located in an almost inaccessible place, when the piece is very hard or soft (tough), and when the piece is very small. In many if not all of these cases, mechanical methods of milling are not completely satisfactory and new methods are needed.

The aircraft industry at the present time has devised a partial solution to the problem with the chemical mill ing process described in us. 2,739,047 for milling aluminurn parts having unusual shapes, e.g., air foils and the like, and where excessive weight must be reduced to a minimum. However, that process is not totally suited for the easily oxidized metals, e.g., iron and the like.

At present, no chemical milling process, that is, the dissolution of metal from selected areas (excluding, of course, electropolishing), is known which has been commercially successful in the chemical milling of iro'n, steel, bronze and brass, the more common of the useful metals. Etching processes are used to engrave metal surfaces, or to reveal microstructures, and pickling is widely practiced, but none of these should be included in the term milling when referring to the controlled removal of a specific amount of metal.

In an attempt to find a satisfactory method for effecting the desired chemical action, the prior art has proposed several methods. One such method utilized the common mineral acids which attack iron. However, this method was not a commercial success because the acid attack is not uniform and leaves a pitted and etched surface rather than a smooth shiny surface.

Another prior art proposal for chemically milling iron, steel and the like utilized a solvent consisting of an aqueous system, e.g., sodium hydroxide (Na+OH-). This was also disadvantageous because the aqueous phase provided an electrically conducting path for an anodic-cathodic attack (which is a mechanism of ordinary corrosion) and prevented proper controls from being effected for the process.

Since iron and steel are the most widely used structural materials, a need exists for a process with which these materials may be eiiectively milled utilizing chemical means. The present invention satisfies this need.

Because of the importance of the irons and steels, the present invention will be described relative to these materials, although it is understood that many of the same considerations will apply equally to other structural materials such as the iron alloys, brass, bronze, copper, zinc, titanium, and even tungsten and molybdenum.

The present invention is predicated upon the discovery of an excellent chemical milling process utilizing a hubbling oxidant, to effect the controlled erosion of metal ice values from iron, steel and like structural materials, and a nonaqueous solvent consisting essentially of phosphorus oxychloride for dissolving the metal values removed from the materials and maintaining the values and the oxidant in a state of solution.

Accordingly, one of the prime objects of the present invention is to provide a chemical milling process which obviates the inherent disadvantages of the prior art systems including the noncontrollable. ancillary corrosion attendant to the aqueous mediums with which these prior art systems operated.

Another object of the present invention is to provide a method of chemical milling in which the coaction between a controlled how of an oxidizing fluid through a nonaqueous solution consisting essentially of phosphorus oxychloride is utilized to control milling depth and to provide an even uniform surface to the metal being processed.

Another object of the present invention is to provide a method for the controlled erosion of metal values from metal stock utilizing a nonaqueous heated solvent consisting essentially of phosphorus oxychloride.

Another object of the present invention is to provide a method for the controlled erosion of metal values from metal stock utilizing a nonaqueous liquid solvent through which is bubbled a controllable stream of a fluid oxidant selected from the group consisting of chlorine, bromine and iodine, whereupon the exposed metal values are removed from metal stock at a rate which is functionally related to the concentration of the oxidant in solution as determined by the controllable rate of flow of the fluid oxidant.

These and still further objects as will appear are fulfilled by the present invention in a manner which is readily discerned from the following detailed description of this invention.

Broadly speaking, the present invention is based upon my discovery that exceptionally fine results are obtained from a chemical milling system having a solvent of phosphorus oxychloride. My process embraces the immersio'n of cast iron, steel and like materials into the solvent and then bubbling a stream of a fluid oxidizing agent through the solvent to mill the surface of the metal at a controlled rate.

A further aspect of my invention is that it can also be adapted for selective milling or embossing by merely applying a protective coating to those portions of the metal article where milling is not desired. In my quest for a coating which would protect the metal by efiectively resisting the action of my milling system, I found that sodium silicate quite unexpectedly provides a coating having all of the aforedescribed properties and an excellent mask in the practice of this aspect of my invention. Thus when I selectively paint or coat portions of the metal articles with sodium silicate, the sodium silicate eifectively inhibits the milling reaction from aifecting the metal over which the sodium silicate is painted.

In one practice of the present invention, a piece of Armco iron is immersed into a heated to C.) phosphorus oxychloride solution. Through this solution I then bubbled a continuous stream of chlorine gas. I permit the action to continue for approximately one hour. During the course of the process, the piece of iron lost 15% of its weight and its thickness decreased about .006

inch. The removal of the iron was quite uniform and.

the surface appeared very shiny and smooth. The corrotion products dissolved in the hot phosphorus oxychloridebut as the phosphorus oxychloride was cooled to about 60 C., the corrosion products precipitated out and were removed from the solution by filtration. The system was then ready for another run. I

In another practice of the present invention a piece heated bath for about one hour.

of 18-8 stainless steel is immersed into a bath consisting essentially of phosphorus oxychloride which has been heated to slightly over 95 C. A stream of bromine is then directed into the solution with slight agitation or stirring. The rate of corrosion productremoval is again approximately .005 inch per hour.

In another practice of the present invention, a piece of 12% chromium steel is immersed into; a bath of phosphorusoxychloride which has been heated to atlcast 90 C. A stream of chlorine was bubbled through the After an hour, the piece of steel was withdrawn from the system. The; prdnot again was found to have very uniform removal and an extremely shiny even surface.

An additional practice of the present invention comprises the immersion of ordinary cast iron into a heated (90-100" C.) bath of phosphorus oxychloride and hubbling a stream of iodine therethrough. Although the bubbling oxidant itself creates sufficient turbulence to provide excellent results and a reasonable rate, it was found that by imparting slight mechanical agitation to the bath while the oxidant is bubbling therethrough, gives equally satisfactory results at a more rapid rate. As in the earlier practice, the cast iron surface was milled ata rate of approximately .005 inch per hour which was main.- tained fairly uniform over a period of two hours.

The uniformity of depth and finish provided upon the piece was substantially comparable with the other pieces which were milled in accordance with my invention.

A further examination of all the samples prepared in accordance with this invention indicated an average surface roughness of X10 inches (R.M.S.) which in the trade is considered to be an excellent surface finish.

In another practice of the present invention, a design is painted on a piece of Armco steel with a sodium silicate solution. The coated piece is then dried at approximately 110. When the painted piece. is dry, it is immersed into a solution of phosphorus oxychloride through which is bubbled a controllable stream of fluid chlorine. When the milling action has continued as long as necessary to obtain the desired depth of metal removal (about 0.005"/hour with a saturated solution at 90 C.), the

electron bands (usually d) of the metal being milled."

The rate of the-reaction of the chlorine in the phosphorus oxychlonide is increased by using the chlorine under pressure to increase the concentration of the 'chlorine in solution. Raising the temperature will also result in a faster reaction rate although the rate of attack is found to increase moreslowly with rising temperatures. It is noted, however, that the solubility 'Qf a gas in a liquid will decrease with temperature thereby decreasing concentration and slowing the reaction-rate. The ran. tive degree of these effects, i.e., heat on reaction rate per se and on concentratiomshould be balanced so as to achieve an optimum-rate of milling. 'Itisfbund-that the maximum rate of removal can occur at a temperature of about 85-100 C; which is less than the boiling point of the phosphorus 'oxychloride. II further found that the appli ation o p s e e m nates some hat the bad shee of decreasingsolubili y h niu rnased temperature although extremely goo r sult a e obtained under a pressure of. one atmosphere.

. 4 herein described are included to exemplify and not limit the invention and that any equivalent formulations are intended within the spirit of this invention especially as consisting of chlorine, bromine and iodine, until the surface has been milled its desired depth; and withdrawing said structural member from said solvent.

2. The chemical process for milling preselected portions of structural members consisting of ferrous metals and alloys comprising: immersingthe preselected portion of the structural member into a heated nonaqueous solvent consisting essentially of phosphorus oxychloride; bubbling through the solvent a fluid oxidant selected from a group consisting of chlorine, bromine and iodine, until the surface has been milled its desired depth; and withdrawing said structural member from said solvent.

3 The chemical process for milling preselected portions of structural members consisting of ferrous metals and alloys comprising: immersing the preselected portion of the structural member into a nonaqueous solvent consisting essentially of phosphorus oxychloride; bubbling through they solvent a fluid oxidant consisting essentially of chlorine until the surface has been milled its desired depth; and withdrawing said structural member from said solvent. v

4. The chemical'process for milling preselected portions of structural members consisting of ferrous metals and alloys comprising: immersing thefpreseleoted portion of the structural member into a heated nonaqueous sol vent consisting essentially of phosphorus .oxychloride; bubbling through the solvent a fluid oxidant consisting essentially of chlorine, until the surface has been milled its desired depth; and withdrawing said structural member fromsaid solvent.

5. The chemical process for milling preselected portions, of structural members consisting of ferrous metals and' alloys comprising: immersing the preselected portion of the structural member into a heated nonaqueous solvent consisting essentially of phosphorus'oxychloride; bubbling a constant stream of chlorine until the surface has been milled its desired depth; and withdrawing said structural member from said solvent.

6. The chemical process for ,rnilling preselected portions or structural members consistingof ferrous metals and alloys comprising: immersing the preselected portion of the structural rnernb er into phosphorus oxychloride heated to a p -timeles t a it b il n po bu bl s a :cc s antstream o ch o ine unt th u f c h en mil ed ts d ired dentha an i hdr g aid u a m b r fro saidsql en A pro es ec icall mi li g met i memb rs otnnris nsp ep rin a a h. as ins essentia ly c ph c oru ext/chl ride; h Ms ai bath t a emperature of fro 9.; nam ng-a met l ic mem e nt said heated bath; introducing a stream of an oxidizing ui s l d rom he roup cons s ing, 0 b om n -;iodin and l r n in o s d'z at f r d t us e gas mcut withsaid member until said member has beenmilled a preselected depth; and withdrawinglhe milled metallic m mb f om dbath- 8. A process for producing amilled metallic member c ord nsto p de rm n d d s gn h ving rai P lion n s n-port n th p es ompr in pa n ing with sodium silicatethe .surface'portions of attunmilled metallic member correspondingto the raid port ons of t e pred te mi ed de i n; d i sa s d silicate; immer ing theso ur cat p inte m t :member into a heated bath consistingessentially of phospm a...

phorus oxychloride; flowing and maintaining a stream References Cited in the file of this patent of a fluid oxidant through said bath for engagement with UNITED STATES PATENTS the unpainted surface portions of said member until said portions become the sunken portions of the predeter- 2756373 Houtz July 1956 mined design having a preselected depth; and Withdraw- 5 FOREIGN PATENTS ing said member from Within said bath. 356,371 Germany Dec. 7, 1921 

1. THE CHEMICAL PROCESS FOR MILLING PRESELECTED PORTIONS OF STRUCTURAL MEMBERS CONSISTING OF FERROUS METALS AND ALLOYS COMPRISING: IMMERSING THE PRESELECTED PORTION OF THE STRUCTURAL MEMBER INTO A NONAQUEOUS SOLVENT CONSISTING ESSENTIALLY OF PHOSPHEROUS OXYCHLORIDE, BUBBLING THROUGH THE SOLVENT A FLUID OXIDANT SELECTED FROM A GROUP CONSISTING OF CHLORINE, BROMINE AND IODINE, UNTIL THE SURFACE HAS BEEN MILLED ITS DESIRED DEPTH, AND WITHDRAWING SAID STRUCTURAL MEMBER FROM SAID SOLVENT. 